The present invention relates to a sheet supplying device for sequentially separating sheets on a stacker and feeding a sheet to a processing platen for reading or printing an image, and a method of detecting overlapping of a plurality of sheets while the sheets are being fed.
A sheet supplying device sequentially supplies sheets stacked on a stacker to a processing platen of a device such as a printer, a copier, or a scanner. An image reading apparatus such as a scanner feeds documents on a stacker to a platen one by one, so that a photoelectric converting device reads an image on the document.
When such a device separates sheets on a stacker one by one and supplies the sheet to the processing platen, if a plurality of sheets (documents) is overlapped and fed (double feed), an erroneous processing may be executed at the processing platen. Accordingly, it is necessary to accurately separate the sheets into a single sheet and detect the double feed of the sheets before the sheet reaches the processing platen, so that the processing is stopped or processing data such as reading information is discarded not to be sent to a processing device such as a printer.
A conventional method of detecting the double feed of the sheets includes an ultrasonic sensor or a photo-sensor for detecting attenuation in an ultrasonic wave or an intensity of light passing through the sheet, thereby determining whether there is a single sheet.
Japanese Patent Publication (Kokai) No. 10-257595 discloses an ultrasonic sensor for detecting a sheet. The ultrasonic sensor includes a piezoelectric oscillation plate such as piezoelectric ceramic at a wave transmission side. A pulse voltage with a predetermined frequency is applied to the piezoelectric oscillation plate to generate oscillation, thereby transmitting ultrasonic waves. A similar oscillation plate is provided at a wave reception side for receiving the ultrasonic waves and converting to an electrical signal. Electric energy is compared with a reference value, thereby determining a single sheet or several sheets.
Japanese Utility Model Publication (Kokai) No. 06-49567 proposes a structure in which a wave transmission element and a wave reception element are arranged opposite to each other between a downstream roller and an upstream roller arranged with a predetermined distance in between, thereby making it possible to detect the double feed while the sheet is in a stable condition. More specifically, with such a structure, the double feed is detected while the downstream and upstream rollers nip the sheet in a straight position during transportation. Accordingly, it is possible to accurately detect the double feed since a leading edge or a trailing edge of the sheet is not curved or does not flip vertically.
When the ultrasonic sensor or optical sensor is used to detect the overlapping of the sheets, if the sheets have different quality, a thickness, or a size, it is difficult to accurately determine whether one or more sheets are being fed at a time. That is, when several sheets contact tightly with one another due to humidity or other environmental factors, it is difficult to determine between a single sheet having a large thickness and overlapped several sheets. When sheets with various sizes are overlapped and shifted in a longitudinal direction, it is difficult to determine between a single sheet having a large size and several sheets overlapped in the longitudinal direction. Moreover, when the sheets are flapped in a vertical direction at a position of the sensor, a transmitted quantity of sound wave or light varies, thereby making it difficult to accurately determine the double feed.
Japanese Utility Model Publication (Kokai) No. 6-49567 has proposed that the overlapping of the sheets is detected while the pair of the rollers supports the sheets. However, it is still difficult to detect the double feed when the sheets tightly contact with one another. Further, when the double feed is detected over a predetermined length to determine that the sheets are shifted in the longitudinal direction, the trailing edge of the sheets flaps upon leaving from the roller, thereby causing a misdetection.
In view of the problems described above, an object of the present invention is to provide a sheet supplying device that can accurately detect the overlapping of sheets even when the sheets tightly contact with one another or are shifted in the longitudinal direction thereof.
Another object of the present invention is to provide an image reading apparatus and a method of accurately detecting the overlapping of original documents while the sheets are fed from a stacker to a reading platen.
Further objects and advantages of the invention will be apparent from the following description of the invention.